1. Field of the Invention
The present invention relates to a receiver having an intermediate frequency such as a single conversion system and a Low-IF (low-IF) system, and more specifically, to a receiver using an IF band-pass filter and an FSK detector.
2. Description of Related Art
In recent years, size and power consumption have been required to be decreased in communication devices such as an ISM (Industry-Science-Medical) band or a specified low power radio, and many functions have been IC-processed. Similarly, receivers used with them also have function circuits such as an LNA, a mixer, an IF filter, an oscillator, a PLL (Phase Locked Loop) frequency synthesizer, a demodulator or the like that are made into IC. Further, in the receiver mainly used in a remote keyless entry (RKE)/tire pressure sensor (TPMS: tire pressure monitoring system), many IF filters have been incorporated. In accordance with this, the IF frequency has been made lower from conventional 10.7 MHz with an external ceramic filter, to several hundreds of kHz.
On the other hand, the reception sensitivity has become higher at high speed. Along with this, the optimization of the IF filter bandwidth and the high sensitivity of the detector are one of the most important tasks in forming a receiver. Especially in a case of the FSK receiver, as shown in FIG. 5, frequency deviation is different among customers, systems, or destination countries. As shown in FIG. 5, a deviation (Δf) of the frequency from a reference value (f) due to the variation of the operation condition is 30 to 50 kHz in Japanese manufacturers; on the other hand, it is only several kHz in Korean manufacturers, for example, which is widely different between countries.
Further, there are configurations in which a transmitter includes a SAW (surface acoustical wave) resonator having low frequency stability (±several hundreds of ppm) which requires low cost and a crystal resonator having high frequency stability (±several tens of ppm) which requires high cost. With these various specifications and configurations, a bandwidth optimization method of the IF filter and an optimization method of the modulation sensitivity of the FSK detector have become more and more important in order to realize high reception sensitivity in one FSK receiver. FIG. 6 is a graph for describing the modulation sensitivity of the FSK detector. The horizontal axis indicates a frequency deviation, and the longitudinal axis indicates an output voltage difference of the FSK detector. In this case, the inclination indicates a modulation sensitivity. When the modulation sensitivity is low, the output voltage difference ΔV is made small, which results in difficult detection. However, when the modulation sensitivity is too high, the detectable range becomes smaller.
FIG. 7 shows a passband control device disclosed in Japanese Unexamined Patent Application Publication No. 2007-158780. An FSK receiver 201 includes a mixer 202, a BPF 203, an A/D converter 204, a HPF 205, a LPF 206, a detector 207, a DC detector 208, a VCO (voltage controlled oscillator) 209, and a filter coefficient selection part 210. First, the FSK receiver 201 mixes an RF signal input from outside through an antenna with a local oscillation signal from the VCO 209 by the mixer 202 to generate an IF signal. After cutting off undesired frequency bandwidth by the BPF 203, this IF signal is subjected to digital conversion by the A/D converter 204. Then, the FSK receiver 201 cuts off the low-frequency component of the IF signal by the HPF 205 and cuts off the high-frequency component by the LPF 206.
Subsequently, the FSK receiver 201 detects the IF signal in the detector 207 to obtain a detection signal (FIG. 8(A)). Then, an automatic frequency control (AFC) operation is performed on the DC component of the detection signal detected by the DC detector 208 so as to make the center frequency of the IF signal match the center frequency fc of the HPF 205 and the LPF 206, as shown in FIG. 8(B). Then, the FSK receiver 201 changes the filter coefficient of the HPF 205 and the LPF 206, and sets an ideal bandwidth that substantially matches the spectrum of the IF signal, as shown in FIG. 8(C).
Further, Japanese Unexamined Patent Application Publication No. 7-58654 discloses a receiver that automatically controls additional circuit means of a receiver in an optimum setting state by discriminating a reception mode from reception signals. According to this related receiver, the reception signal is converted to a first intermediate frequency signal and bisected, one of them is converted to a first demodulation signal through a wide-band-pass filter and a variable band width filter. An AGC means is provided in the reception system, and the first demodulation signal is output as a low frequency through a tone adjustment means. Also, the other one is converted to a second demodulation signal through a narrow-band-pass filter. An interference condition is discriminated in an interference discrimination means from the levels of the first and second modulation signals. The signals passing through the wide-band-pass and narrow-band-pass filters are subjected to fast Fourier transform in first and second Fourier transformation means. A central arithmetic means discriminates the reception mode from the pattern of the Fourier-transformed signal in the condition without interference and controls the variable band width filter, the AGC means, and the tone adjustment means corresponding to it.